It has long been a desire to hermetically seal heat transfer or cooling systems for power generating equipment, such as internal combustion engines (e.g., to positively seal the vent and fill caps), to thereby isolate the liquid coolant and the liquid-side surfaces of the engine and cooling system components from the engine's ambient atmosphere. An ideal such system would have to be truly hermetically sealed and therefore, under normal operation, would never allow the transfer of air, or moisture, into or out of the heat transfer or cooling system. The pressurized cooling systems currently in use represent only a partial step toward this condition because the characteristics of the aqueous-based coolants typically used in such systems do not allow for operation of the system in a hermetically-sealed condition.
With reference, as an example, to current production fuel cells and internal combustion engines, a typical aqueous-based cooling system is pressurized during operation by (i) thermal expansion of the coolant, and (ii) water vapor generated as a result of localized boiling of the coolant within the coolant chambers. These types of cooling systems must therefore be equipped with pressure-relief valves, usually mounted within the fill cap, which limit the maximum system pressure to about one atmosphere (14 to 15 psig) above ambient pressure. When the pressure-relief setting of a valve is exceeded, thermally-expanded coolant and gases or vapors within the system are purged out through the relief valve and into an overflow reservoir having a vent open to the ambient atmosphere. A recovery valve is also provided to permit the coolant in the reservoir, along with ambient air to be drawn back into the coolant chambers when the engine cools down.
In some cases the fill cap, relief valve and recovery valve are mounted on the top of a pressure-resistant overflow reservoir so that during engine operation the entire cooling system, including the reservoir, is pressurized. Thermally-expanded coolant, gases and vapors are purged into the reservoir, which raises the liquid level and in turn compresses the liquid-free space, if any, within the reservoir, and thereby raises the pressure of the entire cooling system. When the system pressure exceeds the pressure-relief valve setting, the gases, vapors, and in some instances, liquid coolant, are purged from the reservoir into the ambient atmosphere. Here again, when the engine cools down, ambient air is drawn back into the cooling system through the recovery valve.
Accordingly, both of these types of systems suffer from the recurring exchange of gases and/or vapors between the engine cooling system and ambient atmosphere during each temperature cycle of engine operation. In addition, there is the continuous problem of water loss caused when small amounts of water vapor (which in some instances includes coolant) are purged through the relief valve and into the ambient atmosphere. Gradually, as small amounts of water are continuously purged from the cooling system, the total coolant volume is reduced and the coolant mixture is changed from the desired mixture to one having a lesser concentration of water. Engine cooling systems for motor vehicles typically employ a liquid coolant which is a 50/50 mixture of ethylene glycol and water (i.e., 50% ethylene glycol and 50% water). As the water concentration in such coolant mixtures is reduced, the greater concentration of ethylene glycol causes the coolant mixture to have a lower specific heat value.
In contrast to their different freezing points, the saturation (boiling) temperature and condensation characteristics of commercially available 50/50 ethylene glycol and water (EGW) heat-transfer liquids or coolants are similar to those of 100% water. The saturation temperature of water is the same as its maximum condensation temperature, 100.degree. C. (212.degree. F.) at 0 psig, and 115.degree. C. (239.degree. F.) at 15 psig. Similarly, a typical 50/50 EGW mixture boils at about 107.degree. C. (224.degree. F.) at 0 psig, and about 124.degree. C. (255.degree. F.) at 15 psig. Water, however, has a much higher vapor pressure than does ethylene glycol, and thus when a 50/50 EGW mixture is boiled the vapor generated is primarily water (about 98% water by volume).
Accordingly, at each system pressure for which a 50/50 EGW coolant produces water vapor, the condensation point for the vapor generated (about 98% water) will be substantially lower than the boiling point of the 50/50 EGW coolant at which it was generated. For example, as indicated above, in a system employing a 50/50 EGW coolant at 15 psig, the water vapor that is generated at about 124.degree. C. (255.degree. F.) will not condense within the coolant chambers until it is entrained within liquid coolant having a bulk temperature of about 115.degree. C. (239.degree. F.) or less. Thus, in order to condense the water vapor, the radiator and/or other heat exchange components of the cooling system would have to establish a heat exchange rate creating a temperature differential (.DELTA.T) of about 8.degree. C. (16.degree. F.) across the engine. However, because motor vehicles are subjected to a variety of operating loads and/or ambient conditions, it has proven to be difficult to control typical internal combustion engines to achieve a heat-exchange rate (.DELTA.T) of more than about 4.4.degree. to 5.5.degree. C. (8.degree. to 10.degree. F.). As a result, during engine operation at high loads and/or ambient temperatures, the EGW coolant temperature frequently approaches the saturation temperature of water at the respective system pressure. The water vapor that is produced cannot therefore be condensed quickly enough to prevent it from occupying a large space within the cooling system, which in turn increases the system pressure and causes substantial volumes of gas, vapor, and in some instances coolant, to be purged through the relief valve.
In an effort to maintain the saturation and condensation temperatures of the bulk coolant relatively high, and in turn minimize the exchange of gases and/or vapors with the ambient atmosphere through the relief and recovery valves, the pressure-relief valves are typically set at about one atmosphere (14 to 15 psig) or higher in order to maintain the cooling systems at such pressures during engine operation. One of the drawbacks of these types of cooling systems, however, is that the relatively high operating pressures, and pressure cycles encountered with shifts in coolant temperatures, place undesirable internal load conditions upon the components of the cooling system (i.e., the radiator, hoses, heater core, clamps, valves, gaskets, etc.), which can in turn lead to leaks and other problems causing system failure.
Another problem encountered with such systems is that the coolant is exposed to relative high amounts of oxygen in the engine's ambient atmosphere. The introduction of oxygen into the coolant causes an increasing rate of oxidation of the coolant, and in the production of acids (oxsolic, acetic, etc.) and thus significantly limits the effective useful life of the coolant additives. This is discussed in further detail in my co-pending application Ser. No. 08/449,338, entitled "A Method Of Cooling A Heat Exchange System Using A Non-Aqueous Heat Transfer Fluid", which is hereby expressly incorporated by reference as part of the present disclosure.
My U.S. Pat. No. 5,031,579, dated Jul. 16, 1991, which is hereby expressly incorporated by reference as part of the present disclosure, shows a condenserless apparatus for cooling an internal combustion engine with a substantially anhydrous, boilable liquid coolant having a saturation temperature above that of water. The apparatus comprises a coolant chamber surrounding the cylinder walls and combustion chamber domes of the engine, and a coolant pump which is adapted to pump coolant through the coolant chamber at a flow rate so that the liquid coolant substantially condenses the coolant vaporized upon contact with the metal surfaces of the engine.
The apparatus of the '579 patent further comprises means for exhausting gases and/or vapors from the coolant chamber which is coupled in fluid communication with the chamber at a location at or below ambient pressure. The means for exhausting preferably includes a conduit coupled on end to the coolant chamber, and an expansion tank coupled to the other end of the conduit for receiving the gases and/or vapors from the coolant chamber and purging the gases through an outlet port into the ambient atmosphere. The liquid within the expansion tank is maintained at a level above the tank's connection to the conduit in order to provide a liquid barrier between the coolant chamber and the engine's ambient atmosphere.
The apparatus of the '579 patent further comprises a dehydrating unit coupled in fluid communication with an outlet port of the expansion tank for dehydrating the ambient air drawn into the expansion tank and thereby minimizing the exposure of the coolant to ambient vapors. Thus, an engine equipped with this type of apparatus can limit the amount of moisture returning to the coolant chamber by employing both the liquid barrier in the expansion tank and the dehydrating unit. The high vapor pressure of water will cause any water in the expansion tank to vaporize at higher ambient temperatures (above about 32.2.degree. C. or 90.degree. F.) typically stabilizing at a water content of about 2% to 5%, and the dehydrating unit will in turn maintain the coolant at a lower moisture level (about 1% to 2%) during its effective life.
The apparatus of the '579 patent can use substantially non-aqueous coolants operating at ambient vent pressures, and therefore derives significant benefits over currently produced engine cooling systems. However, although the dehydrating unit provides significant advantages, it may be perceived in certain applications as being relatively bulky and thus undesirable. In addition, even when the engine is not running, the dehydrating unit will continue to absorb moisture, and thus requires periodic maintenance to remain effective. The preferred coolants in the apparatus of the '579 patent are forms of diols (e.g., propylene glycol) and are basically hygroscopic such that if exposed, they will continue to absorb water vapor. If the dehydrating unit becomes saturated, it will permit moisture to pass into the expansion tank and in turn expose the coolant to undesirable levels of moisture. Thus, particularly at low ambient temperatures (e.g., below about 10.degree. C. or 50.degree. F.) the liquid barrier in the expansion tank will not function to completely prevent the introduction of water vapor into the engine coolant chamber, but rather will absorb a certain amount of moisture. In addition, the thermally-expanded coolant received in the expansion tank would be exposed to the ambient atmosphere and higher levels of oxygen, thus increasing the oxidation rate of the coolant, and in turn limiting the effective life of the coolant additives, as described above.
Accordingly, it is an object of the present invention to overcome the drawbacks and disadvantages of the above-described cooling systems for internal combustion engines and other power generating equipment.